Year: 2026
Type: preprint
Abstract: Earth's core, made mostly of iron, makes up about 32% of the planet's mass. In the giant-impact theory of the Moon's origin, a roughly Mars-sized planet, often called Theia, struck the young Earth and placed iron-poor rocky debris, mostly from the impactor, into orbit, where the Moon formed. The Moon has only a small iron core. What that impact did to the share of Earth's mass held in its own core depends on more than the impact itself, because two processes, each supported by published work, push in opposite directions. Collisions between bodies that already have iron cores can knock away rock more easily than iron, which raises the core's share. But models of Earth's core formation suggest that, as the planet grew, more of its iron combined with oxygen and stayed in the rocky mantle, which, this Commentary argues, lowers the core's share. Earth's iron-to-magnesium ratio is about 10% above the solar value, which has been interpreted as a net loss of about 10% of rock relative to metal over Earth's whole growth. A simple estimate suggests that the rock removed to form the Moon would raise Earth's core share by only a few percent of its value at most. Because the two processes oppose each other and other factors are unknown, the net change caused by the Moon-forming impact cannot yet be stated with confidence.
Author: T A Vijayakumar
Language: English
Topic: Astro and Planetary Science
Subfield: Space and Planetary Science
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7222837736